A dynamic random access memory (DRAM) includes an array of memory cells with each memory cell having a capacitor and a transistor. As DRAMs become more highly integrated, the size of the capacitor and the operating voltage both decrease. Because a predetermined charge is required-on the capacitor to discriminate between logic levels, reductions in the size of the capacitor may be limited by the capacitance required to store the predetermined charge. The electrical charge Q of a capacitor is determined by multiplying the capacitance C by the operating voltage V. Accordingly, in order to store a predetermined charge at a lowered operating voltage, the capacitance of the capacitor must be increased.
The capacitance of a capacitor can be increased by increasing the effective area of the capacitor, by increasing the dielectric constant of the dielectric layer, and by decreasing the thickness of the dielectric layer. Because increasing the effective area of a capacitor may require an increase in the size of the capacitor, and because decreasing the thickness of the dielectric layer may be limited by manufacturing constraints, these approaches to increasing capacitance may not be sufficient. The use of dielectric materials having higher dielectric constants may, however, provide increased capacitance while reducing the size of the capacitor without requiring a dielectric thickness which is unnecessarily difficult to produce. For example, Ta.sub.2 O.sub.5 can be used to produce a dielectric film having a significantly higher dielectric constant than a dielectric film of the same thickness formed from silicon oxide SiO.sub.2. The film formed from Ta.sub.2 O.sub.5 can have a dielectric constant on the order of 20 to 25. Accordingly, when using a Ta.sub.2 O.sub.5 film as the dielectric layer of a capacitor, the surface areas of the capacitor electrodes can be reduced without reducing the capacitance and without significantly increasing manufacturing costs.
A flowchart for a method of forming a capacitor including a Ta.sub.2 O.sub.5 dielectric film is illustrated in FIG. 1. A method for forming a capacitor includes the steps of forming a lower electrode of the capacitor S1, cleaning the lower electrode surface to remove a naturally occurring oxide film therefrom S2, rapid thermal processing S3, depositing the Ta.sub.2 O.sub.5 film S4, annealing the Ta.sub.2 O.sub.5 film with ultraviolet light and ozone (O.sub.3) S5, annealing the Ta.sub.2 O.sub.5 film with oxygen (O.sub.2) S6, and forming an upper electrode for the capacitor S7.
The step of rapid thermal processing S3 eliminates an oxidation barrier generated at an interface between the lower electrode and the Ta.sub.2 O.sub.5 film. The rapid thermal processing step may be provided as a succession of heat treatments. The step of annealing the Ta.sub.2 O.sub.5 film with ultraviolet light and ozone (O.sub.3) S5 reduces oxygen vacancies in the Ta.sub.2 O.sub.5 film. The step of oxygen (O.sub.2) annealing S6 reduces weak spots in the Ta.sub.2 O.sub.5 film.
It is known, for example, to reduce leakage current of a Ta.sub.2 O.sub.5 film by annealing the film. An annealing method is described in a publication by Shinriki et al. entitled "UV-O.sub.3 DRY-O.sub.2 : Two-Step Annealed Chemical Vapor-Deposited Ta.sub.2 O.sub.5, Films For Storage Dielectrics Of 64-Mb DRAM's", IEEE Transactions On Electron Devices, Vol. 38, No. 3, March 1991, pp. 455-462, the disclosure of which is hereby incorporated herein by reference.
Typically, a deposition system is used to deposit the Ta.sub.2 O.sub.5 film, and separate systems are used to anneal and clean the structure. Accordingly, the Ta.sub.2 O.sub.5 dielectric film may be exposed to air resulting in the adsorption of water and/or free carbon. Accordingly, exposure to air may cause deterioration of the dielectric layer and the adjacent electrodes. In addition, the loading and unloading of the production substrates from one system to the next may result in unnecessary heating and cooling between each step as well as unnecessary transfers of the material thus reducing the efficiency of the process and lowering the throughput.